Vehicle automatic transmissions are controlled by a shift lever which operates, through a notched detent plate, a spool valve which for every drive position, including reverse, of the shift lever directs power to the vehicle wheels. For parking, the shift lever is moved from or past reverse to park position, to place the spool valve in a non drive position and operate a mechanical linkage system including some form of cam means for effecting movement of a pawl towards a toothed parking gear. Should the pawl first engage the surface of a tooth, as is the usual case, rather than the space between two teeth, yielding reslient means must be provided in the linkage to enable the shift lever to be moved to its full park position while the pawl is held out of its park position due to its engagement with a tooth. After the vehicle has drifted a short distance sufficient to align an inter-tooth space with the pawl, the resilient means acts to drive the pawl into its park position.
A recognized problem with the above described arrangement is that when a driver carelessly moves the shift lever to a position intermediate the reverse and park position and the engine is running, the resilient means can anchor on the pawl while it is temporarily hung up on a tooth surface and drive the system from a partial parked condition back to reverse causing the vehicle to suddenly move in reverse. Persons standing behind the vehicle can and have been seriously injured.
Efforts to combat the foregoing problem have involved the re-design of the gates or slots for the shift lever in the driver's compartment with the intention of inducing the driver to move the lever into its full park position. These efforts, however, have not been successful and accidents to person and property continue to occur.
In my co-pending applications Ser. No. 363,080 filed Mar. 29, 1982, and Ser. No. 485,513 filed Apr. 15, 1983 I disclose arrangements for combating the foregoing problem, not at the driver or shift lever end of the system but at the transmission end, the arrangements being such that as the linkage system is moved a predetermined distance towards park, means are provided for exerting a force on the system, first, to restrain it from moving towards reverse and, second, to urge the system towards the full park position so that even if the driver has been initially careless in positioning the shift lever in what he thought to be park, the system not only cannot move unexpectedly to reverse but it will be driven into the full park position as soon as the pawl aligns with a space between two teeth.
The object of the present invention is to provide an improved system for accomplishing similar results as in the foregoing applications but in which the resilient means, which normally yields when the pawl engages a tooth surface, is arranged to operate in only the direction of park and never in the direction of reverse.
More particularly, it is an object of the invention to provide a system of the foregoing nature wherein the resilient means is, for every position of the system except park, stressed away from park and when the system is moved in the direction of park, the stress exerts a force on the system tending to drive it towards park but never towards reverse.